When Ameonna was first released there was discussion that her special ability is useless. Pic for reference:
There were concerns about her low defensive stats, and she can’t heal while in ghost form. Think of it instead as she has the highest attack stat of any 4*, and purple doesn’t hit very hard in the 4* category. Rigard, Sabina, cyprian, the only hitter is tiburtus. If you’re stacking a purple team against a titan, or competing in an event, even if she dies look at the damage she contributes to your purple tiles! The ghost ability seems wonky but it’s really quite fun when the titan or AI on a defensive raid wastes on attack on her while she can’t take damage. Plus her boost on purple tiles while in ghost form.